Use of microwave ovens with power available on recreational vehicles has been proposed. U.S. Pat. No. 4,667,075 to Sakurai, for example, proposes powering the oven by using three-phase, AC power generated by a vehicle mounted generator.
Microwave ovens employ magnetrons which require high power for short periods of time. Typically 1 kw. or more of electrical power is needed for cooking food during intervals that usually range from one to seven minutes depending upon what needs to be heated. Such power demands currents that can approach 100 or more amperes from a 12 volt DC battery when that is the source of power for the microwave oven.
It is imperative in the use of microwave ovens that microwave energy not be allowed to escape. Extensive leakage controls are applied. One such control involves interlocks that sense when the oven door is opened and thus prevent operation of the magnetron. It is not practical to operate such interlocks while low voltage high current electrical power for the magnetron is being passed through. At the typical DC input voltage of 12 volts used in cars, boats and other transports the electrical current is too high to safely pass through conventional microwave interlocks. The door operated switches would be likely to arc and present sufficient electrical resistance to overheat.
In view of the high power needs of a microwave oven, it is desirable to operate it from regular AC line voltage when that is available. This requires an ability to operate both from AC and DC power. In one known AC or DC powered microwave oven, the POWAMATE, sold by E.C. Smith & Sons of Bedforshire, England, an inverter is used to generate the full AC power needed to operate the oven from a 24 volt DC source. This AC power is then applied through the oven interlocks to operate the magnetron. The oven is built on top of the inverter, and as a result, is large, cumbersome and expensive, and difficult to install on a boat with limited storage capacity.
In another DC powered microwave oven sold by Peter Schiff Enterprises from Cookeville, Tenn., a conventional microwave oven is converted to DC battery operation. An inverter is used to generate power for the magnetron. DC power is supplied through a high current fuse to a center-tapped winding of an output transformer as well as through thermostat switches, a door interlock and a timer-controlled switch to a printed circuit (PC) board and to a DC-powered fan and light. A 60 Hz. signal is generated on the PC board to operate a timer motor.
This type of microwave oven cannot operate from AC power, and appears to employ the interlocks in a manner that is different from conventional AC powered microwave ovens.